Veneering is one of the oldest woodworking arts, one that has been practiced for centuries. It is an economical way of covering a large area with expensive wood. Through the use of modern techniques, wood veneer panels are used extensively in the furniture making industry.
One problem which has plagued the veneering art not only in centuries past, but also in present day furniture manufacture is the checking of veneer panels, that is, the formation and propagation of cracks along the wood grain as veneer panels age. Veneer is nothing more than extremely thin wood, wood that is typically very much different in terms of porosity and density than the substrate to which it is laminated. Thus thin veneer surfaces are not only subjected to the stresses imposed during typical high pressure and high temperature laminating processes, but they are subjected to the internal stresses resulting from differential response of the substrate and the veneer overlay to environmental changes such as temperature and humidity. With time the thin veneer can yield to those internal stresses by forming checks or cracks, typically along with the veneer grain which propagate as the internal stress is relieved.
There has been a significant effort in the industry to define veneer cutting, laminating, processing and finishing techniques which improve veneer resistance to checking. Thus, it is generally known that sanding veneered surfaces helps to reduce checking by reducing internal stresses. It has also been reported in the literature that nitrocellulose and melamine based veneer finishes tend to be more brittle and consequently more likely to crack than, for example, polyester or polyurethane base finishes. Most U.S. furniture manufacturers use polyester based finish formulations for veneer surfaces. Polyurethane coatings do not find widespread use in the furniture industry because of several negative factors including, but not necessarily limited to, cost, ease of use and ease of repair.
In spite of the many recent developments in coating technology there exists a significant need, especially in the furniture finishing industry, for development of improved composition and methods for providing check resistant veneer finishes of consistent quality.
Accordingly, one object of this invention is to provide an improved veneer finishing process particularly adapted for commercial furniture manufacture.
Another object of the invention is to provide a surface reactive, nitrocellulose-modified urethane-based wood finishing composition.
A further object of this invention is to provide a veneer finishing system which provides enhanced resistance to veneer checking.
Those and other objects are accomplished in accordance with this invention by substituting a surface reactive, urethane-based finishing composition for standard wash coat and/or sealer coat formulations in commercial finishing schedules to provide finished veneer surfaces which exhibit markedly enhanced resistance to checking. The surface reactive compositions are formulated to have a solids content comparable to wash coat and sealer coat compositions specified in typical commercial furniture finishing schedules. The present surface reactive compositions comprise nitrocellulose, an organic polyfunctional isocyanate, preferably an aliphatic diisocyanate, and an organotin catalyst in an aprotic solvent mixture. Preferably the composition further comprises a synthetic resin in a weight ratio of about 1:4 to about 4:1 relative to the nitrocellulose component.
In accordance with this invention a surface reactive composition is used as the first film-forming coating applied to the veneer surface during the finishing process. Following standard finishing schedule procedures, but substituting the present reactive composition for a standard wash coat composition, a veneer surface is wash-coated with a surface-reactive coating composition and thereafter subjected to light sanding. The exposed wood fibers can be subjected to further staining or treatment with pigment glaze prior to applying a sealer coat, preferably itself a surface reactive composition of this invention, to the veneer surface. The surface finish is then completed in accordance with the remaining finishing schedule specifications. Thus, the present veneer finishing system can be substituted into an existing furniture finishing schedule without modification of the end steps of the process, i.e., the application of top coats and the rubbing and polishing operations, steps which are important to production of consistent finishes characteristic of the specified finishing schedule.